The present invention relates to dc motor optimizing systems and, more particularly, to optimizing systems for dc motors having separately excited armature and field windings.
Heavy duty material handling vehicles provided with electric motors typically use a lead-acid battery that can weigh many thousands of pounds. Besides providing the energy source to the vehicle, in many instances the battery also provides vehicle counterbalance.
The ratio of the load weight to the gross unloaded vehicle weight of industrial lift trucks is extremely important. For example, if an unladen vehicle weighs 12,000 lbs, and the maximum load weight it can carry is 4,000 lbs, then the gross unladen/laden weight may vary from as little as 12,000 to as much as 16,000 lbs. This represents a change of 33% in motor torque requirements. Moreover, the vehicle must be able to maneuver on loading ramps, further increasing the motor torque requirements. For these and other reasons, a control system capable of extracting precise and efficient work from the vehicle is desirable.
The main motive element of this type of vehicle, referred to as the traction system, usually consists of a series-wound dc motor coupled to a gear reducer and drive wheel.
The rotational direction of the series-wound dc motor is controlled by the polarity orientation of the field winding with respect to the armature. Generally, the field winding orientation is controlled through a pair of contactors, such that when power is applied across the field-armature combination, the motor is caused to rotate in the desired direction.
The series-wound dc motor, heretofore used extensively in industrial lift trucks, features one very important characteristic: it has high torque at zero speed. This is extremely useful in providing the necessary starting torque.
Typically, the field-armature combination is controlled as a single unit. Motor speed regulation is most often achieved through voltage switching utilizing such power semiconductor technologies as silicon-controlled-rectifiers (SCRs). The voltage drop associated with the SCR as well its duty cycle limit impose a speed limit on the motor.
However, a series dc motor may operate only along its characteristic commutation curve limit. Since changing torque loading arises from variations in load capacities, travel path conditions and grade variations, motor speed variations occur.
With the proper controls, the use of a shunt-wound dc motor under independent field and armature control can provide distinct advantages over conventional series-wound dc motors for lift truck applications.
The separately excited dc motor represents a highly coupled multi-input, multi-output, non-linear, dynamic system or plant. It is highly coupled in the sense that, when one of its inputs is changed, all of the outputs are affected. This is undesirable, since the purpose of control is to knowingly and intentionally affect the desired output(s) only, without altering other output states.
U.S. Pat. No. 4,079,301 issued to Johnson, III discloses a dc motor control circuit having separately excited armature and field windings. The control circuit is operable in both the constant torque and constant horsepower modes. The transfer characteristics of the circuit provide high gain at low frequencies and low gain at higher frequencies. The circuit can further reduce the gain at low frequencies when motor operation switches from the constant torque mode to the constant horsepower mode.
U.S. Pat. No. 3,694,715 issued to Van Der Linde et al discloses a contactless dc motor reversing circuit. The current from a variable frequency, pulsed dc source is applied to the series field by a pair of solid state switching devices for forward motor rotation. A second pair of solid state switching devices applies current for reverse motor rotation. Common to both switching devices is a third switching device which normally carries the induced armature current between pulses. It is de-energized during transfer of conduction between both pairs of switching devices, assuring that the blocking state of one pair occurs before the second pair is turned on.
U.S. Pat. No. 4,264,846 issued to Sauer et al discloses a speed control braking circuit for a dc motor. The field and armature currents are independent of each other to allow motor operation in the field weakening region. The armature current is set by a pulsing dc element. The field winding is contained in a series circuit with a switch which is connected in parallel with the dc element. Shunted across the field winding is a field current bypass diode.
U.S. Pat. No. 5,039,924, issued to the present applicant and hereby incorporated by reference, discloses a system for optimizing control of separately excited shunt-wound dc motors, where optimization is achieved through microprocessor-based independent pulse-width-modulation (PWM) control of a chopper (armature) and an H-bridge (field). Connected to the armature is an armature voltage amplifier for varying the applied armature voltage. A field voltage amplifier is also provided for varying the voltage applied to the field winding. A first sensor is connected to the motor armature in order to determine the motor rotational speed. A second sensor is connected to the armature circuit in order to determine the armature current. A third sensor is connected to the field circuit in order to determine the field current. An optimal controller uses the motor speed, field current and armature current information, and adjusts the armature voltage and the field current.
Unfortunately, monitoring motor speed is an indirect method of determining the speed of the vehicle itself. Moreover, since the speed of the traction motor is not always linearly proportional to the actual speed of the vehicle (e.g., when turning), attempting to control the motor based solely on the speed thereof is not necessarily the most accurate vehicle speed control method.
Moreover, connecting or attaching an encoder to the armature of the motor is often problematical. Not only must space considerations be taken into account, but heat dissipation techniques must be employed.
It would be advantageous to provide a system that optimizes for motor losses.
It would also be advantageous to provide a motor optimizing system capable of producing variable torque while maintaining constant speed.
It would also be advantageous to provide a system for controlling a motor based on data representative of motor speed, but not to require motor speed measurement at or near the motor itself.
It would also be advantageous to provide a system for controlling a motor that is based on direct speed measurement of the wheel(s) driven by that motor.
It would still further be advantageous to provide a system in which the optimizing control is achieved using software.